1. Field of the Invention
This invention relates to containers, adapted for holding food, which are suitable for use in microwave oven cooking.
2. Description of the Prior Art
It is known in the art to produce various shaped articles from foamed and unfoamed thermoplastic materials such as polystyrene sheet by thermoforming methods. Many such articles are containers used for packaging foods.
Many thermoplastic containers, however, are melted by hot fats and cannot be used to package fat-containing foods that are to be heated in a microwave oven. In particular, polystyrene containers are not suitable for microwave cooking. Similarly, containers made from poly(p-methylstyrene) are not resistant to hot fats when heated in a microwave oven.
Microwavable thermoplastic containers are described in copending U.S. Pat. No. 4,335,181 of John P. Marano and Maya S. Farag. The containers disclosed in that patent are made from polymers which are not in themselves microwavable in the presence of hot fats, for example, polystyrene, poly(para-methylstyrene) and polyethylene. Melt-through in the presence of hot fat is avoided by laminating to the substrate a protective film such as polyacrylonitrile or polyethylene terephthalate.
Containers made from poly(para-methylstyrene) which are resistant to hot fats under microwave cooking conditions are described in U.S. Pat. No. 4,205,114 to Peter J. Canterino and Harold A. Arbit. Fat resistance is obtained by cross-linking the poly(para-methylstyrene) with ionizing radiation.
Articles such as plates which are microwavable in contact with fatty foods are commercially available. Some of these are believed to be polystyrene foam with a layer of impact polystyrene on their inner surfaces. The impact layer is relatively thick (5 to 8 mils) and the rubber content of the impact polymer is about 3 percent.
In accordance with this invention, it has been found that significantly less of the relatively expensive impact polymer can be used if the rubber content of the impact polymer is greater than about 5 percent without sacrificing resistance of the article to hot fat under microwave conditions. It has also been found that minimizing orientation of the impact layer is beneficial in obtaining good resistance to hot fat in a microwave oven. Thus, a microwavable article can be made having other desirable characteristics more cheaply than previously thought possible.